Ester-substituted diaryl carbonates, such as bismethylsalicylcarbonate, are important reactants in the production of polycarbonate resins. As the use of polycarbonate resins has increased, the efficient production of ester-substituted diaryl carbonate has become more important.
Waste streams from the ester-substituted diaryl carbonate manufacturing process require special handling for disposal. The waste streams typically comprise a variety of compounds such as ester-substituted diaryl carbonate, ester-substituted phenol, and derivatives thereof. Currently there is no economically useful process to efficiently recover these compounds from a waste stream.
As is reported in U.S. Pat. Nos. 7,339,070 and 7,151,189, processes exists for recycling waste product streams from non-ester-substituted diaryl carbonate production facilities, for example those that produce diaryl carbonates such as diphenyl carbonate (DPC). In these patents a process is disclosed where a waste product stream containing diphenyl carbonate is reacted in an alkyl alcohol, such as methanol, at high temperatures, high pressure, and high residence times to breakdown residual diphenyl carbonate into its phenolic precursor, phenol. Phenol is then separated from the balance of the components and used to form/reform diphenyl carbonate.
The present Inventors found that this process does not produce an acceptable stream of ester-substituted phenol that can be used for later reactions to form/reform ester-substituted diaryl carbonate and polycarbonate. At these high temperatures, high pressures, and high residence times, the Inventors found the ester-substituted phenol present in the reaction mixture converts to acid-substituted phenol which has been found to cause problems in down stream reaction processes that produce ester-substituted diaryl carbonate and polycarbonate from the ester-substituted diaryl carbonate. In addition, such conditions are very energy intensive and require special equipment and thus have high investment and variable costs.
As is reported in U.S. patent application Ser. No. 11/748,951, it was found that an acid-substituted phenol such as salicylic acid can lead to process instability in the melt formation of polycarbonate using the ester-substituted diaryl carbonate as a carbonate source. It is believed that the acid-substituted phenol negatively impacts the performance of the melt transesterification catalyst used in the melt polymerization process. The acid-substituted phenol is believed to have its greatest impact at the earlier lower temperature stage of the melt polymerization process, for example during the oligomerization stage.
Problems arising from the acid-substituted phenol and the generation of it are not observed in the methods described in U.S. Pat. Nos. 7,339,070 and 7,151,189 because non-ester-substituted diaryl carbonates, such as diphenyl carbonate, do not contain a degradable ester linkage that is capable of degrading to form the undesired byproduct acid-substituted phenol.
Therefore, it would be extremely desirable to find processes where a waste stream from an ester-substituted diaryl carbonate production facility can be efficiently recycled to recover reusable materials from the stream while minimizing or eliminating the formation of acid-substituted phenol in the product stream.